1. Field of the Invention
The invention relates to the management of hydrocarbon production. More particularly, the invention relates to methods and apparatus for the long term monitoring of reservoir fluids by locating instruments in wells.
2. State of the Art
In an oil field, numerous holes are drilled into the formation before and after oil production begins. Some holes are discovery, appraisal, and delineation wells to determine the location and boundaries of a petroleum reservoir. Other holes will become injection and production wells. Many of the hole sections which are not used for production (e.g., all except the injection and production wells) are usually sealed with cement. The injection and production wells are, in most parts of the world, each completed by installing a casing in the hole and surrounding the casing with cement. The casing is usually made from metal pipe sections having a diameter close to that of the well hole. The sections are lowered into the well hole and cement is poured into the annulus between the casing and the formation. The casings are then perforated at predetermined depths where it is believed that an oil reservoir is located.
Water may be naturally present in the reservoir acting on the oil to urge it out through the well bore. Often, water (or steam) is injected into the reservoir from an injection bore located near the production bore. As oil is extracted from the well, the water moves through the porous media of the formation closer to the production well. As a result, the oil-water interface changes shape. If the location of the oil-water interface is not monitored during production, it is possible that the well will produce a mixture of oil and water. In some cases, it is possible for the well to produce more water than oil. Similarly, it may be desirable to monitor the oil-gas interface should one exist.
Well logs are a primary source of information used to map the distribution of fluids in hydrocarbon reservoirs. The logs are made from various sensor measurements such as resistivity, pressure, temperature, sonic velocity, etc. It is a common practice in a producing well to interrupt production and re-enter the well with wireline logging tools in order to ascertain changes in the reservoir which occur as a result of production. This practice is undesirable because it interrupts production and it is expensive. In addition, logging tools used inside a production casing can only measure satisfactorily water saturation and can not measure satisfactorily changes in formation pressure, a measure which is very useful in determining the efficiency with which fluids are being extracted from the formation. Furthermore, in a completed sub-sea well, re-entry into the well with logging tools may be either impossible or prohibitively expensive.
U.S. Pat. No. 4,475,591 discloses a method for obtaining pressure measurements in a producing well by permanently mounting pressure sensors in the cement filling between the formation and the casing. Placement of these sensors is often difficult. When the sensors are located in the cement surrounding the casing, there is a danger that the sensors or the cables to the sensors will be damaged during installation. Damage can also occur during casing perforation or side tracking.
U.S. Pat. No. 5,214,384 discloses a method for electronic self-potential logging in a cased observation well which is located between a steam injector well and a production well. A metallic casing complicates the method and the placement of electrodes in a casing or between a casing and the formation is also difficult as mentioned above. Furthermore, this method requires the drilling of an additional well which will not produce any hydrocarbons.
Co-owned U.S. Pat. No. 5,467,823 discloses a method for long term monitoring of a reservoir by locating a pressure gauge in an observation well which traverses the reservoir. The upper portion of the observation well is cased and the remainder is left open. A measuring device is suspended from a cable and lowered into the observation well to the depth of the reservoir. The measuring device includes a pressure gauge mounted in a casing-like tube section and an explosive perforation device. Cement is injected into the observation well to a depth corresponding to the reservoir. After the cement cures, the explosive device is ignited to perforate the cement and put the reservoir in fluid communication with the pressure gauge. While this method has advantages over the previously described methods, the cable is still in danger of damage as the well is being injected with cement, and the sensor package can be damaged during the perforation explosion.